1. Field of the Invention
The present invention relates to a novel, useful multicellular monolithic ceramic body which is referred to as a ceramic honeycomb and a preparation thereof.
2. Description of the Prior Art
Discharge gas of internal combustion engines such as automotive emission contain noxious components such as carbon monoxide and hydrocarbons. The gas causes air pollution together with discharge gases from the other industrial apparatuses. In view of prevention of public pollution, it is necessary to convert the noxious components into non-noxious components. It has been considered that a catalyst device is one of the most effective devices.
As a catalyst for cleaning the automotive emission etc., it has been proposed to use a pellet or grain type catalyst or a multicellular monolithic catalyst having many gas passages partitioned by thin walls in one or two direction. The latter, multicellular monolithic ceramic body has remarkable advantages of low pressure loss of the emission gas, high wearing resistance under severe vibration and light weight and compact and has the improved durability. Therefore, the demand has been increased. However, the multicellular monolithic ceramic body has not enough thermal shock resistance in comparison with the pellets in view of the structure. Therefore, it is necessary to form it by a refractory ceramic having low thermal expansion and contraction and low expansion coefficient as the material. This fact is one of serious problem for using such ceramic honeycombs in this application.
As materials for the ceramic honeycombs, it has been proposed to use alumina-silica, alumina, zirconia-alumina, zirconia-magnesia, mullite, zirconia-silica (zircon), zircon-mullite, titania, magnesia-alumina spinel, zirconia and the other special non-oxide type ceramics such as Si.sub.3 N.sub.4 and carbon. However, it has not been practically used except the following magnesia-alumina-silica (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 :cordierite), lithia-alumina-silica(Li.sub.2 O.Al.sub.2 O.sub.3.nSiO.sub.2 ; n:2-8, .beta.-spodumene), because of high thermal expansion coefficient and the oxidative consumption at high temperature. It has been highly required to improve the catalyst in view of severe condition in applications. There is substantially not any hope for the improvement.
The cordierite has been widely used because of low expansion (thermal expansion coefficient of 0.12 to 0.3% at 1000.degree. C.) and relatively high stability at high temperature (low decomposition) as the characteristics of ceramics.
The above-mentioned .beta.-spodumene has the thermal expansion coefficient (-0.1 to -0.2% at 1000.degree. C.) lower than that of cordierite. The temperature for the allowable application is too low as lower than 1200.degree. C. In the practical condition requiring higher heat resistance, .beta.-spondumene has not been practically used. In feature, there is substantially not possibility to use it.
The cordierite honeycombs have been practically used, however, they have certain disadvantages.
One is the requirement of higher heat resistance, because of improvement of engines, and instantaneous high temperature (caused by back-fire etc.) and long durability. The other is the requirement to large thermal shock resistance.
On the other hand, the following physical characteristics are required for the fabricated product as the ceramic honeycomb as a carrier for a catalyst. That is, high porosity is required for carrying the catalyst and high strength of the substrate is required because of the requirement of thinner walls of the ceramic honeycomb for effectively using the expensive catalyst. It is not easy to satisfy both characteristics in view of the balance that higher porosity results in lower strength. The problem has not been dissolved.
It has been studied to overcome all or most of the disadvantages of the conventional products and to develop an improved ceramic honeycomb and has been succeeded to develop a satisfactory ceramic honeycomb.